The invention relates to recovery of cooking chemicals in pulp production processes based on organic chemicals, particularly in processes based on formic acid and acetic acid. In the process of the invention recovery of cooking acids is arranged to be carried out in conditions which enable production of more cooking acids from dissolved hemicellulose and lignin included in the used cooking liquor and simultaneous release of acids bound to the dissolved material by de-esterification to compensate for cooking chemical losses. At the same time furfural is formed, which is utilized as an extractant in the recovery of cooking chemicals by distillation.
The chemicals used in pulp production have to be recovered and reused as efficiently as possible for environmental, legislative and economic reasons. In conventional pulp production processes, where wood material is used as the raw material, recovery of cooking chemicals has been rather successful. One has also succeeded in reducing the amount of sulphur compounds in exhaust gases. In several countries, however, the limited availability of wood material constitutes a problem when production is to be increased, and thus attempts have been made to replace wood with non-wood fibre sources. However, in such straw pulp processes recovery of chemicals has not been successful. So far it has not been economically feasible to implement a pulp plant that would function according to the “total effluent free” principle.
In pulp production processes based on organic chemicals, efficient recovery of chemicals is particularly important because organic chemicals are typically more expensive than inorganic chemicals and thus efficiency of their recovery has a considerable influence on the economy of the whole process.
So far prior art processes based on organic chemicals have involved considerable chemical losses. Neither have the chemical recovery processes been that simple and inexpensive. The best-known pulp production processes based on organic compounds include Alcell, Organocell, Milox and Formacell processes.
In the recovery of chemicals the object is to enable as efficient and economic re-use of chemicals as possible without loading the environment.
The recovery of chemicals in pulp production processes based on organic acids and alcohols has been described e.g. in WO 93/15261 (Lora J. H. et al.), EP 0 584 675 A1 (Nimz, H. H. H. & Schöne, M.) and in Pohjanvesi, S. et al., Technical and economical feasibility study of the Milox process, The 8th International Symposium on Wood and Pulping Chemistry, Jun. 6-9, 1995, Helsinki, vol. 2, pp. 231-236. In these prior art processes the recovery of acids is based on thermal separation processes and distillation.
Vapour-liquid equilibrium between organic acids and between furfural and organic acids as well as separation by distillation are described e.g. in Hunsmann, W. & Simmrock, K. H., Trennung von Wasser, Ameisensaure und Essigsäure durch Azeotrop Destillation, Chemie-Ingenieur-Technik 38 (19), 1966, pp. 1053-1059 and in Tsirlin, Yu. A., Studies of Liquid-Vapour Equilibrium in the System Furfural-Water-Acetic Acid, Zhurnal Prikladnoi Khimii 35 (1962), no.2, pp. 409-416.
Finnish application 980995 (WO 99/57364) (Chempolis Oy) describes a pulp production process based on formic acid and acetic acid and a process for regenerating cooking acid by evaporation and distillation. The concentrated cooking liquor is evaporated in a multi-phase evaporator to a concentration of dissolved solids of 50 to 80%, and water is distilled from diluted acids by means of overpressure to the typical total concentration of formic acid and acetic acid, i.e. 80 to 90%, and this mixed acid is returned to cooking.
Finnish application 973474 (WO 99/105959) (Chempolis Oy) describes a process for the recovery of chemically bound formic acid from pulp. The process utilizes free formic acid.